This invention relates to an improvement in the hooking arrangement for two-needle lockstitch sewing machines More particularly, this invention relates to the relative position between needles and hooks in two-needle lockstitch sewing machines.
The relative positions of the needles and hooks in a conventional two-needle lockstitch sewing machine are shown in FIGS. 5a and 5b. FIG. 5a is a plan view of the needles and hooks and FIG. 5b is a front view of the needles K.sub.1 denotes a left hook and K.sub.2 denotes a right hook. KK.sub.1 and KK.sub.2 denote left and right hook points, respectively N.sub.1 and N.sub.2 denote left and right needles, respectively. T.sub.1 denotes a left needle thread and T.sub.2 denotes a right needle thread. Arrows in FIG. 5a show the rotational direction of each hook K.sub.1 and K.sub.2 respectively.
In the above-described needle and hook arrangement, each needle thread's passageway is preferably symmetrically arranged and each hook hooks a thread loop formed on the outer side of the needle. Thus, the stitching for each needle thread is performed.
FIG. 6 is a plan view illustrating the relation between the direction of threading and the stitching S.sub.1, S.sub.2. During stitching S.sub.1, S.sub.2, the squeezing (displacement of twist) direction for the needle thread when it passes through the needle eye is different for each of the needles N.sub.1, N.sub.2 .
Generally, Z-twist threads are used for sewing machines The displacement of twist by Z-twist thread due to the edge effect of the needle eye is shown in FIG. 7.
Referring to FIG. 7, each thread is a Z-twist type and each left and right needle N.sub.1, N, is rotated by 90 degrees counter clockwise around the needle axis from the original position of FIG. 5b to illustrate the displacement of twist for each needle In this drawing, the displacement of twist for each needle thread is exaggerated for illustration.
As illustrated, when each needle thread passes through the needle eye, the squeezing direction is different for each of the needle threads Thus, the direction of displacement of the twist for each thread is opposite In case of the right needle N.sub.2, the twist advances from the needle eye to the upper side and, accordingly, the stitching S.sub.2 will be performed with the loose-twist thread.
In case of the left needle N.sub.1, on the other hand, the twist advances from the needle eye to the lower side and, accordingly, the stitching S.sub.1 will be performed with the more-twisted thread. Although the left and right thread had a uniformly equal degree of twist when each was set in a bobbin, after stitching with the two-needle lockstitch machine the left stitch line is stitched with the more-twisted thread and the right stitch line is stitched with the loose-twist thread. Thus, stitching is not uniformly performed. In particular, the left needle may be problematic and cause accidental thread cut, or disorderly stitching such as wobbling or balloon stitch (isolated idle loop).
The above described displacement of the twist in the needle thread during stitching with the two-needle lockstitch sewing machine is caused by the edge effect of the needle eye. How this displacement causes problems is described in the report "The Displacement of the Twist in the Needle Thread during Sewing Process", in the "Journal of the Textile Machinery Society of Japan" Vol. 37, pp. 185-193, April, 1984.
To solve these problems, a combination of S-twist thread for the left needle and Z-twist thread for the right thread has been applied. The arrangment of using S-twist thread for a small quantity of workpiece results in higher costs and therefore such application is limited only to where mass production is used.
In another arrangement, the rotational direction of the hooks may be altered to be oppositely directed (one clockwise and the other counter clockwise). This arrangement alone does not solve the problem either Adjustment of the shape of the needle eye edge or loosening of the thread tension have also been tried, but these arrangements are restrictive and have also proved insufficient to solve the problem.